This invention relates to the production of electrical energy by means of a magnetohydrodynamic generator and, more particularly, to a method and system for so producing electrical energy which is well adapted to be powered utilizing heat sources at relatively low temperature levels. Thus, the invention is particularly suited for being powered utilizing solar energy which provides heat, but ordinarily at a lower temperature level.
Magnetohydrodynamic generators are not new. However, operation of such generators, as such has been proposed in the past, has been accompanied with a number of disadvantages and problems. For instance, it has been proposed to utilize as the operating fluid in such a generator an ionized gas or plasma operating at extremely high temperatures, for example, 3000.degree. C., and with the operating temperatures obtained using coal or gasfired heat sources. In addition to the disadvantage of requiring costly energy sources to operate, such systems present corrosion problems and other difficulties by reason of the high temperatures employed. In a magnetohydrodynamic generator system wherein a liquid metal is accelerated through the generator to produce electrical energy, again relatively high temperatures are needed to reduce the more common metals to liquid form and, because the usual liquid metal is quite dense and relatively viscous, power losses are encountered in circulating the metal through the system. More recently, and as disclosed in U.S. Pat. No. 4,191,901, a system has been proposed which utilizes vapors formed by volatilizing an organic liquid, such as Freon, to propel a liquid alloy, such as a liquid sodium potassium alloy, through a magnetohydrodynamic generator. Among the disadvantages which characterize this type of system are the relatively viscous nature of the alloy and power losses that result on propelling such through the generator, the explosive nature of the alloy in the presence of moisture and attendant safety problems, and the fact that the fluid medium passing through the generator is actually a two-phase system comprising the metal alloy liquid and the propelling vaporized Freon gas. The gas in such fluid medium lowers the electrical conductivity of the medium which is passing through the generator and again reduces power output. Additionally, the two phases in the medium, i.e. the gas and the liquid alloy, must be separated on leaving the generator, introducing problems attendant to such separation and the return of such materials prior to recycling through the generator.
Contemplated by the present invention is a magnetohydrodynamic generator system which utilizes a liquid ammonia solution as the medium circulated through the generator, with such medium rendered conductive through the presence in the liquid ammonia of a dissolved element or elements which dissolve and ionize in liquid ammonia to produce cations of the element or elements and electrons which are solvated by the ammonia. The liquid ammonia solution which passes through the generator is what might be thought of as a single phase liquid in the sense that there is not present a mixture of two basically immiscible materials, as disclosed in the above-identified U.S. patent. The medium circulated through the generator possesses excellent electrical and thermal conductivity, relatively low viscosity and relatively low density (as compared, for instance, to a liquid metal), all contributing to efficient production of power when the medium passes through the generator. The solutions contemplated are obtainable at relative low cost. Problems of separation of immiscible materials, as presented in the process of the above patent, are eliminated.
As contemplated in a preferred embodiment of the invention, the liquid ammonia solution, on being returned for recycling through the generator, is pressurized to a pressure at or above the critical pressure of the solution which, for a concentrated lithium ammonia solution, is approximately 1600 psi. The solution may then be heated to its critical temperature, or a temperature near the critical temperature of the solution (which, for the lithium ammonia solution just described, is approximately 200.degree. C.), to produce a sub or super critical fluid of the solution, and a lowering in the density of the solution (by reason of expansion of the solution). It is this expansion of the solution which produces movement of the solution at high velocity through the generator, with attendant production of electrical energy. On being discharged from the generator, the solution may be cooled with a concommitant increase in density, repressurized, heated and recirculated through the generator.
As will be apparent for the above broadly described embodiment of the invention, heating to produce operation of the generator as contemplated herein may be performed at relatively low temperatures. This adapts the system of the invention for being practiced efficiently using relatively low temperature heat sources, such as solar heat, geothermal heat, waste heat, etc.